I've always wondered how much drafting helps/assists you while you ride. And, particularly, I always wondered at what speed you had to be riding for you to notice the effects. Lets discuss the former first. There is no doubt that drafting helps save/conserve energy while riding. How much? Well that depends. That depends on the terrain, the wind velocity and direction, the person (or vehicle) you're drafting and how closely you follow the guy/gal you're drafting.
Let's assume for now, that you're riding on a flat road with no wind. So, the only resistance or drag is the coefficient of drag (Cd) that you feel head on from the still air/wind. I don't have a graph/chart showing the energy savings at different speeds but there is no doubt the faster you go the more energy savings there is. For example, there may be a 40% savings in energy when you're drafting at speeds approaching 30 mph while the savings may diminish to 20% at 20 mph. I'm not sure if the relationship is linear or not. I'm guessing it's pretty close. I can tell you (from experience) that I've been nearly sucked along in the middle of the pack of a 1 mile loop Criterium that averaged 27 mph. I'd say I was averaging no more than 200w maintaining that speed in the middle of the pack. When I was out front of that same Criterium, I've noticed my power output closer to 300w. That's a 50% increase. I've also moto-paced before (behind a motorcycle) at 30 mph and had an interval workout called "pop-outs" where I drafted behind the moto then "popped-out" into the windstream. What a difference? I didn't have a power meter at the time, but I'll bet there was a huge power difference.
What is the magic speed that you need to achieve to take advantage of the energy savings from drafting? Before I try to answer that question, lets make a few assumptions: 1) again the wind/air is still 2) we're on a flat road 3) we're drafting a 6 ft. tall 170 lb rider (trust me, the bigger the rider in front of you the better the drafting advantage or energy savings) and 4) we're drafting at approximately 1 ft. behind the wheel in front of us (no more no less). The further away you are from the rider in front of you the lesser the advantage. From my experience, and from others experience that I've read online, you can benefit at speeds as low as 12-15 mph. Albeit, the benefit is small, lets say 10%. But, I don't notice a benefit until at least 16-17 mph and that benefit is small. I'm guessing a 20% energy savings at that speed. Where I notice the biggest benefit is over 20mph. Again, keep in mind, if I'm drafting a big guy that is 6'4" vs. a gal that is 5'4" I'm going to notice a bigger energy savings with the big guy.
So, the next time you're riding in a group and you're riding in the middle, sucking the wheel in front of you, just remember that you're not doing nearly as much work as the guy out front. i.e. you're not as strong. Don't forget to thank the guy out front after he pulls too. After all, it's hard work out front..but then again, some of you "wheel suckers" out there would never know that since you spend all of your time in the middle of the pack just to say you rode with the Big Dogs!
I'll try to find more accurate numbers in the weeks ahead and I'll revisit this blog. I think it's good information to know.
Until then, Power ON! Coach Rob
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